GMC Lock-up overdrive info.

[mypentium]

Thanks for the interest Don, and I agree with you that a lock-up
overdrive conversion (if possible) would be a real winner for these
coaches. As it stands now, I have no further resources to draw on to
advance the concept. I can only serve as a catalyst to draw attention to
the possibility. Perhaps some of the contributors to this list may be in
a better position to follow up on my search to determine whether such a
conversion is possible or practical.

There may be some owners who have connections to rebuilding, racing, or
manufacturing that might want to take up the torch. IMHO, it might be
worthwhile to further explore the possibility of interchangeability or
adaptation of the required parts from other and perhaps newer GM
transmissions. It may even be possible to modify an existing input
shaft to work with a lockup converter or even to manufacture such a
part. Wouldn't this make a great challenge for someone?

Sorry for the poor copy of my first post from Fri, Nov 6. I will revise
it so that it is easier to read and post it again when I am done this
message.
- -Bill

Date: Sun, 8 Nov 1998 06:49:45 -0000
From: "Donald W. Miller"
Subject: Re: GMC: Lock-up overdrive info.

Bill,

Thanks and your research goes into my keeper file.

I seem to recall the 67 Toronado two speed torque convertor
(JetawayTransmission wasn't it?) has what I consider a pretty serious
flaw.

In the high slippage mode it provides zero torque multiplication.
Although this does let the engine turn faster for more torque and hp, it
produces lots and lots of HEAT while in this mode. Bad news. I
suppose some might call it a six speed, but I would'nt.

On the other hand, the lockup torque convertor provides near optimum
torque multiplication when unlocked and zero slippage and no heat when
locked.
Perfect.

I hope you continue to pursue this because a good lockup torque
convertor should be a winner in the GMC.

Cheers,

Don Miller

75 Glenbrook project
Shenandoah Valley of Virginia

 

[mypentium]

(the following is a revised and more readable copy of my post from Fri,
Nov 6 that may be easier to digest --Bill)

I am a prospective GMC motorhome owner and have followed your mailing
list information with interest for some time. After following the
discussions on power trains, it occurred to me that due to the unique
and discontinued design of the transmission, it suffers from the lack of
a lock up converter; a design innovation that began in the early 80's
when downsizing of power trains began. With an overdrive lockup
convertor and overall 3.66 gearing, there would be plenty of power in
all three gears, and with the .75 transmission gearing O.D. would offer
as opposed to the 1.00 of third gear, my calculations say that you would
be running about 2400 rpm at 70 mph. This is right on the max torque rpm
to provide best mileage, and of course, converter slippage (causing
power loss and transmission heat) is eliminated as in modern
transmissions.

After searching web resources and library material on GM Oldsmobile
transmissions, I found that the TH425 transmission was a modification of
the conventional TH400 transmission, and that many parts were
interchangeable. The torque converter is interchangeable with the TH400
and the TH350 (a smaller series of GM transmissions) and there was a
TH350C variant with a lockup converter. There was also a switch-pitch
torque converter configuration used in them early TH400's and TH425's
(including the '66-'67 Toronado) that allows a selection of two
different stall speeds (you have recently had listings that referred to
this configuration as a six speed transmission).

It appears that the torque converter requires a matching input shaft and
pump with valve to reverse torque converter fluid flow for engagement,
and the valve is solenoid controlled, allowing simple external
activation. With this in mind, I posted the question to alt.autos.gm to
see if such a parts can be interchanged with related and similar systems
for the switch-pitch Toronado. Even though the reply was not
encouraging, this information may be of interest to the group, and maybe
even though as Gary says...'that dog won't hunt', there may be another
way found to 'skin the cat' with group involvement.

I posted this question to alt.autos.gm:

Will a lockup torque converter from a TH350C fit the input shaft of a
switch-pitch equipped TH400? From what info I have, early TH400's used
a hydraulically actuated switch-pitch converter that required a special
input shaft, pump, and solenoid valve; a similar configuration to the
1983 TH350C. These parts perform the same function as those operating
the lockup converter in the TH350C; namely, switching modes of
operation by reversing the oil flow through the converter. If a lockup
converter would fit the switch-pitch input shaft configuration, could it
not then be engaged by the electric solenoid valve through a manual
external circuit (eliminating TCC) with brake, speed, and shifter
lockout switches? Interchangeability of parts between the TH350 and
TH400 series suggests to me that this just might work; and similar
manual circuits have been used to adapt switch-pitch converters for
street use.

Would someone more familiar with TH transmissions please tell me if I am
on the wrong track with this idea? In case you might think it easier to
just replace the transmission with a newer 200-4R or 700-4R, I must add
that my application is the front drive variant of the TH400, namely the
TH425 from a pre-1978 Toronado ( Toronado also had the switch-pitch
converter in 1966-67 ).
- - -Bill

I recieved this reply to my question from a transmission specialist:

Will it fit...yes, partly.

Will it work...no.

After an hour of searching, I finally located a v/p 400 converter and
the pump that matches it. Results: Distance from end of input shaft
recess to rear of pump hub: 400 v/p = approx. 6 inches. 350 l/u =
approx. 5.5 inches. That means the 350C converter will bottom on the
input shaft before the pump lugs engage fully. The oil passage that
controls the variable pitch in the 400 is located on the pump stator
shaft immediately behind the stator splines. The passage for the 350
lock-up is in the end of the input shaft.

Sorry to be the bearer of bad news, it sounded fine in theory, but in
the real world, (as Ross Perot would say) ..that dog won't hunt!.
--Gary

 

[bartz paul]

Bill:

You may want to call Chuck Stoddard, 440-423-0809, who owns Caspro, and
get his opinion on the feasibility.

Paul Bartz

From: mypentium [mailto:mypentium]
Sent: Monday, November 09, 1998 4:32 PM
Subject: Re: GMC: GMC Lock-up overdrive info.

Thanks for the interest Don, and I agree with you that a lock-up
overdrive conversion (if possible) would be a real winner for these
coaches. As it stands now, I have no further resources to draw on to
advance the concept. I can only serve as a catalyst to draw attention to
the possibility. Perhaps some of the contributors to this list may be in
a better position to follow up on my search to determine whether such a
conversion is possible or practical.
There may be some owners who have connections to rebuilding, racing, or
manufacturing that might want to take up the torch. IMHO, it might be
worthwhile to further explore the possibility of interchangeability or
adaptation of the required parts from other and perhaps newer GM
transmissions. It may even be possible to modify an existing input
shaft to work with a lockup converter or even to manufacture such a
part. Wouldn't this make a great challenge for someone?
Sorry for the poor copy of my first post from Fri, Nov 6. I will revise
it so that it is easier to read and post it again when I am done this
message.
- -Bill
Date: Sun, 8 Nov 1998 06:49:45 -0000
From: "Donald W. Miller"
Subject: Re: GMC: Lock-up overdrive info.

Bill,

Thanks and your research goes into my keeper file.
I seem to recall the 67 Toronado two speed torque convertor
(JetawayTransmission wasn't it?) has what I consider a pretty serious
flaw.
In the high slippage mode it provides zero torque multiplication.
Although this does let the engine turn faster for more torque and hp, it
produces lots and lots of HEAT while in this mode. Bad news. I
suppose some might call it a six speed, but I would'nt.
On the other hand, the lockup torque convertor provides near optimum
torque multiplication when unlocked and zero slippage and no heat when
locked.
Perfect.
I hope you continue to pursue this because a good lockup torque
convertor should be a winner in the GMC.